Before buying ask for seller to prove that he is account owner by sending you personal message from account you want buy with text you provide.
Certain members are now getting on third party sites, messaging the sellers in this way and then exposing the accounts when they show up here again.
It really, really bothers a lot of people and you'll never know whether your expensive buy will be rumbled until it's too late. There are some extremely skilled detectives out there. It's becoming a no go area.